Showcases of this type must therefore meet requirements of various types, in relation to the preservation and integrity of the exhibits. Moreover, of course, these showcases have to ensure the best visibility for the exhibits.
Systems for making fixed parts integral to one another are therefore very important, so as to ensure the necessary strength to the showcase.
In order to improve visibility, showcase manufacturers seek where possible to use transparent materials—typically glass—for the showcase walls. In addition to ensuring better visibility of the exhibits, the extensive use of glass is often desired by showcase designers since the transparency of the material allows giving maximum emphasis to the exhibits.
Therefore, showcases have been developed with a base surmounted by a case formed by walls; the base accommodates all the necessary technical components to ensure that the environment within the case is protected and is thus normally closed by metal walls, which conceal all technical components from view; vice versa, the case walls are totally or partly made of glass for the above reasons. The technical components can also be accommodated in compartments extending vertically, especially in the case of showcases leaning on walls.
The case may be made of transparent material walls all integral to one another, so as to form a bell that is lifted from the base to access inside the showcase. Or, some walls (fixed walls) are integral to the base and to one another while one or more walls (opening walls) are movable, thanks to suitable opening mechanisms.
In order to make the fixed walls of transparent material integral to one another, it is normal to bond the walls to one another by means of suitable adhesives, along adjacent peripheral edges, often cut at 45°; these junctions are also known as miter-square junctions.
Thanks to modern adhesives, it is possible to ensure good stability and safety of bonding. However, it is sometimes possible that abnormal, unpredictable or unforeseen conditions may lead to a bonding strength lower than expected. This can happen for example because some abnormal and not necessarily detectable condition has occurred in the bonding step which can lead (immediately or maybe later) to a reduction of the adhesion between the glued parts. Or, it may happen that particular environmental conditions (in terms of temperature, humidity and exposure to light or to atmospheric agents) may change—worsening them—the adhesion features of a bonding over time, despite it was made to perfection.
In these cases, the showcase will be exposed to the risk of detachment between the walls, and thus of collapse. This is a risk that occurs mostly in the bonding between a vertical wall and the horizontal upper wall (ceiling), where the weight of the horizontal wall is relieved on the vertical wall through the adhesive, stressing it: in case of failure of the adhesive, the collapse of the horizontal wall right on the works in the showcase at the time of the collapse is immediate and inevitable. Clearly, therefore, it is a risk that, while minimum, should be absolutely avoided: in this case, in fact, instead of protecting the works on display, the showcase would be a hazard itself.
Also, in any case, even the best adhesives have a decay of their grip over time and therefore are only guaranteed for a limited time; when this time limit is reached, replacing the showcase is inevitable, although apparently still intact.
However, in the cases where it is desired to exclude any risks without giving up a showcase with transparent walls, cutting the edges to 45° is usually given up, especially for bonding together a vertical wall and the horizontal upper wall: the latter is glued on top of the vertical wall, leaving the edges simply straight, at 90° with respect to the faces of the walls; these junctions are also known as juxtaposed junctions. In this case, the weight of the horizontal upper wall does not stress the adhesive, reducing the risk of failure; moreover, when failure occurs, the horizontal upper wall is still resting on the vertical wall and therefore does not collapse on the works contained in the showcase.
However, while in a showcase in which the walls are joined at 45° the junction line between the walls is located at the geometrical edge of the of the showcase and is therefore substantially invisible to the observer, in a showcase in which the walls are joined at 90° the junction line between the vertical wall and the horizontal wall is lower than the geometrical edge of the showcase by a distance equal to the thickness of the horizontal upper wall. In this position, the junction line between the walls is clearly visible and can therefore constitute a disturbance to the viewing of the exhibits.
Therefore, there is the problem of totally preventing the risk of collapse of a showcase with glued walls but without introducing any visible elements that may disturb the viewing of the exhibits.